Prompt parser: Account for empty field in alternating words syntax

[catboxanon]

Description

Closes #8161

Updates the prompt scheduler to allow for empty fields to be specified in the alternating words prompt syntax. This means shorthand syntax as mentioned in the issue like [fe|]male works, as opposed to typing out [female|male].

Adds two new prompt tests and all current tests pass (besides test noted # not handling this right now).

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[rubberbaron]

Thanks for this, however, from reading the code, this looks like it wouldn't handle [|fe]male; it allows empty strings everywhere except in the first position.

[catboxanon]

Maybe I should've added a test case for it but it actually does work. Feel free to try for yourself.

[rubberbaron]

You are absolutely correct that it works, my apologies. I don't understand how it works without putting the first case inside [ ], though, but it does! I even printed out the prompt schedule to make sure it was actually parsing correctly, and it's all good.

(I do see in hindsight now that there isn't any ambiguity that complicates LALR parsing after all, so it does make sense that the fix would be straightforward.)